1. Field of the Invention
The present invention relates to a method for fabricating a novel electrode for lithium secondary battery.
2. Related Art
In a lithium secondary battery having been actively researched and developed recently, battery characteristics such as charge/discharge voltages, charge/discharge cycle life characteristics, and storage characteristic are greatly influenced by an electrode used. Therefore, battery characteristics are enhanced by improving an active material used for an electrode.
Although it is possible to constitute a battery having high energy densities per weight and volume by using lithium metal as a negative active material, a problem occurs that lithium is deposited like dendrite to cause an internal short-circuiting.
On the other hand, a secondary lithium battery is reported which uses any one of aluminum, silicon, and tin which are electrochemically alloyed with lithium under charge (Solid State Ionics, 113-115, p.57(1998). Among the above materials, silicon is particularly prospective as negative electrode for a battery with a high capacity, having a large theoretical capacity. For this reason, various secondary batteries respectively using silicon for the negative electrode are proposed (Japanese Patent Laid Open No. Hei10-255768). In the case of the negative alloy electrode of this type, however, a sufficient cycle characteristic is not obtained because the alloy that is an electrode active material is pulverized due to charge/discharge and thereby, the current-collecting characteristic is deteriorated.
In order to overcome these problems, proposed is an electrode for lithium secondary battery obtained by forming a microcrystalline silicon thin film or an amorphous silicon thin film on a current collector through a thin-film forming method such as CVD method or sputtering method, which electrode shows excellent charge/discharge cycle characteristics (International Patent Laid Open WO01/31720A1 etc.).
The above electrode for lithium secondary battery is formed by using a metallic foil as a current collector and depositing a silicon thin film on the metallic foil through the thin-film forming method. However, warping or wrinkle may be produced on the metallic foil, when a silicon thin film is formed on a metallic foil. When warping or wrinkle is present on a current collector, the charge/discharge reaction becomes ununiform and charge/discharge cycle characteristics are deteriorated.
It is an object of the present invention to provide a method for fabricating an electrode for lithium secondary battery capable of preventing warping or wrinkle from being produced on a current collector when depositing an active thin film, and capable of forming an uniform active thin film on the current collector.
The present invention is a method for fabricating an electrode for lithium secondary battery formed by depositing an active thin film on a metallic foil serving as a current collector, in which a tension is applied to the metallic foil so as to pull an area on which the active thin film formed, from the both sides, during the deposition of the active thin film.
According to the present invention, it is possible to prevent warping or wrinkle from being produced on a current collector because of applying a tension to a metallic foil so as to pull an area on which an active thin film is formed, when depositing the active thin film. When depositing an active thin film on a metallic foil through sputtering method or CVD method, the temperature of the metallic foil is increased due to the energy of deposited fine particles. When the temperature of the metallic foil increases, the metallic foil expands and thereby, warping or wrinkle is produced on the metallic foil. The present invention makes it possible to prevent a warping or wrinkle from being produced by the above expansion because of applying a tension from the both sides so as to pull an area on which an active thin film is deposited. Therefore, it is possible to form a uniform active thin film on a current collector.
In the present invention, it is preferable to put a metallic foil on a support. Moreover, it is preferable to put the metallic foil on the support and apply a tension so that the metallic foil closely contacts with the support. By applying a tension to contact with the support closely, it is possible to more securely prevent warping or wrinkle from being produced. Moreover, by bringing the metallic foil in close contact with the support, it is possible to release the heat produced in the metallic foil to the support and thereby, prevents the temperature of the metallic foil from ununiformly increasing.
Furthermore, it is preferable that a device for controlling the temperature of the metallic foil is provided in the support. Thereby, it is possible to further uniform the temperature distribution of the support. Furthermore, the above temperature control device is preferably for cooling the metallic foil. By providing the cooling device, it is possible to efficiently suppress a temperature rise of the metallic foil.
Furthermore, it is preferable that a portion of a support contacting with a metallic foil has a curved shape. Specifically, it is preferable the portion is a convex curved shape. Such a shape improves the adhesion between the metallic foil and the support so as to uniformly control the temperature of the active thin film deposition area of the metallic foil. As a support having such a shape, a roller is exemplified. When using a roller as a support, it may use a continuous type thin-film forming apparatus which allows continuous feeding of a metallic foil and continuous forming of an active thin film on the metallic foil. In this case, a roller may rotate in accordance with the movement of the metallic foil. Alternatively, a roller-shaped support which does not rotate may be used.
An active thin film of the present invention is formed by depositing the film on a metallic foil. As a method for forming an active thin film by depositing the film, preferably used is a method in which a raw material is supplied from a gaseous phase to form the thin film. This type of the method includes sputtering, CVD, vacuum evaporation, and thermal spraying methods.
An active thin film of the present invention is a thin film composed of active material capable of absorbing and releasing lithium. An active thin film absorbing lithium by being alloyed with lithium is preferably used. As a material for the above active thin film, silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, gallium, and indium are given as examples.
It is preferable to use an active material mainly containing silicon or germanium from the viewpoint that it is easy to form a thin film by the thin-film forming method from gaseous phase. Moreover, from the viewpoint of a high charge/discharge capacity, it is particularly preferable to use an active material mainly containing silicon. It is preferable that an active thin film is an amorphous thin film or a microcrystalline thin film. Therefore, an amorphous silicon thin film or microcrystalline silicon thin film is preferably used as an active thin film. The amorphous silicon thin film is a thin film in which a peak around 520 cmxe2x88x921 corresponding to a crystal region is not substantially detected through the Raman spectroscopic analysis, and the microcrystalline silicon thin film is a thin film in which both a peak around 520 cmxe2x88x921 corresponding to a crystal region and a peak around 480 cmxe2x88x921 corresponding to an amorphous region are substantially detected through the Raman spectroscopic analysis. Moreover, it is preferable to use an amorphous germanium thin film, a microcrystalline germanium thin film, an amorphous silicon-germanium alloy thin film, and a microcrystalline silicon-germanium alloy thin film.
Although a metallic foil used as a current collector of the present invention is not restricted as far as the foil can be used as a current collector of an electrode for lithium secondary battery, it is preferable to use a metallic foil made of a metal not alloyed with lithium. A copper foil or nickel foil is used as this type of the metallic foil.
We have already found that when using a copper foil as a metallic foil and depositing a silicon thin film as an active thin film, the copper component in the copper foil diffuses in the silicon thin film and a mixed layer is formed at the interface between the copper foil and the silicon thin film. We have also found that when the copper component properly diffuses in the silicon thin film, a solid solution of copper and silicon is formed in the mixed layer and thereby the adhesion between the copper foil and the silicon thin film is improved. Furthermore, we have found that when the copper component is excessively diffused in a silicon thin film, the intermetallic compound of copper and silicon is formed and the silicon thin film is easily peeled from a copper foil. The difference between these diffusion states also relates to the temperature when depositing a silicon thin film. Namely, when the temperature of a copper foil excessively increases, copper excessively diffuses. Therefore, by bringing a metallic foil into close contact with a support and suppressing a temperature rise of the metallic foil, it is possible to improve the adhesion between the active thin film and the metallic foil.
Accordingly, a fabricating method of the present invention is particularly useful in the case that the temperature of the metallic foil increases and the component of the metallic foil diffuses in the active thin film to form a mixed layer at the interface between the metallic foil and the active thin film. According to the present invention, it is possible to suppress a temperature rise of the metallic foil and control the diffusion of the metallic foil component in the active thin film.
The thickness of a metallic foil of the present invention is not restricted, but it is preferable that the foil has a thickness of 5 to 40 xcexcm as a current collector of an electrode for lithium secondary battery.